Field of the Invention
This invention is concerned with catalytic cracking processes wherein coke containing catalyst is stripped prior to coke burn-off.
The field of catalytic cracking has undergone progressive development since 1940. The trend of development from the Thermofor catalytic cracker (TCC) to the widespread use of the fluid catalytic cracking (FCC) process is evident from both patent and technical literature. The trend in FCC processes has been to all riser cracking, use of zeolite-containing catalysts, heat balanced operation, and complete combustion of CO to CO.sub.2 within the regenerator.
Although new production facilities favor the installation of FCC units, existing TCC units are currently in use throughout the world. A conventional TCC unit is described and illustrated in U.S. Pat. No. 4,473,658, the disclosure of all of which is herein incorporated by reference. Although the TCC process varies from refinery to refinery, all installations use 4 to 20 Tyler mesh size particles of a high surface area, inorganic porous acidic solid as catalyst; all operate in the absence of added hydrogen; all utilize a continuous feed of the stream to be cracked, as well as a continuous cyclical transport of an active inventory of catalyst between a regenerator zone and a cracking zone.
Major trends in FCC and TCC processing have included modifications to accommodate feedstocks that contain more metals and sulfur and attempts to improve catalyst stripping.
Higher sulfur levels in the feed, combined with a more complete regeneration of the catalyst in the regenerator, increase the amount of SO.sub.X in the regenerator flue gas.
Refiners have long known that conventional primary strippers are somewhat inefficient and leave large amounts of potentially recoverable or strippable hydrocarbon on the spent catalyst. They know that permits 10-50 percent of the burning in an FCC regenerator is of potentially recoverable cracked product, rather than catalytic coke. Although the industry has long known this problem, no one has come up with a completely satisfactory solution.
One of the constraints of operating a conventional, primary FCC stripper is the pressure balance. Usually the FCC stripper is at the base of a reactor vessel. Frequently spent catalyst is stripped in an annulus about the riser reactor. The pressure above the stripper is the same as the pressure at the reactor riser outlet. It is not possible to operate the reactor stripper at a pressure lower than the reactor outlet, because both share a common vapor space and must operate at the same pressure.
It is not possible to simply provide an extra, secondary stripper operating at a low pressure intermediate the primary stripper and the regenerator. Although it is theoretically possible to take material from the primary stripper and discharge it into a secondary stripping vessel, and operate this secondary stripping vessel at a reduced pressure, it would not be possible to easily transport catalyst from this mythical, reduced pressure, secondary stripper to the FCC regenerator.
Accordingly, the art has not developed a secondary stripper which operates at a reduced pressure relative to both the FCC reactor and to the FCC regenerator.
Some attempts have been made to reduce the amount of sulfur fed to the regenerator of an FCC unit by improving stripping. In U.S. Pat. No. 4,481,103, which is incorporated by reference, the spent catalyst was contacted with steam at about 500.degree. to 700.degree. C. for about one-half to 10 minutes, preferably 1 to 5 minutes, in the absence of oxygen, during a stripping step. This became known as the "long residence time stripper." After this stripping, the stripped catalyst was regenerated with an oxygen-containing gas at FCC regeneration conditions sufficient to convert most of the coke deposited on the catalyst to carbon dioxide to produce a regenerated catalyst.
A high temperature stripper is described in U.S. Pat. No. 4,424,116 which is incorporated by reference. A spent, coke-laden catalyst is mixed with a portion of hot regenerated catalyst then passed through a first stripping zone comprising an elongated chamber, which is at least in part vertical or inclined, for a residence time sufficient to remove some of the hydrocarbons on the spent catalyst.
The mixture of regenerated and spent catalyst is then separated from the gaseous stream, and introduced into a second stripping zone and again contacted with a stripping gas. After the second stripping operation the catalyst is again separated from the stripping gas and regenerated. The regenerated catalyst is quite hot, being about 650.degree. C., preferably at least about 675.degree. C., more preferably 705.degree. C. and most preferably at least about 720.degree. C. Thus, the temperature difference between the regenerated and spent catalyst will be at least about 55.degree. C. up to about 195.degree. C.
The stripping gas, recovered from the second stripping zone stripper is preferably combined with the cracked product from the reactor. This restricts the operating pressure of this secondary stripper to about the riser top pressure.
U.S. Pat. No. 2,965,454 (Harper) discloses a down flow dilute phase catalytic cracking process, followed by further dense bed cracking of heavy feed (including liquid) in a dense bed reaction zone labeled as a "first stripper". Spent catalyst withdrawn from the "first stripper" was carried up a riser with steam and discharged into a second stripper where occluded hydrocarbon conversion products are stripped with stripping steam. Stripped catalyst then flowed horizontally into a catalyst regenerator, which operated at essentially the same pressure and at the same elevation as the second stripper.
The second stripper 23 would operate at a reduced pressure relative to the "first stripper" 11, but the "first stripper" 11 would not function as a true stripper because it functions as a dense bed cracking zone wherein heavy liquid is cracked to lighter products with resulting deposition of coke upon catalyst therein. The second stripper would operate at a lower pressure relative to the "first stripper", and the second stripper would be sorely needed because cracked products are generated in the "first stripper" 11 and would be present in the "stripped catalyst".
Another limitation on the approach described in the '454 patent is that the second stripper operates in pressure balance with the regenerator 29. If an attempt were made to operate the second stripper 23 at a lower pressure than the regenerator 29, it would not be possible to get catalyst to flow from the second stripper 23 via line 28 into regenerator 29. This constraint, namely that the second stripper must operate at a pressure essentially equal to that of the regenerator, limits the benefits obtainable by such a process.
Thus, the prior art attempts at removing hydrocarbons from spent catalyst in a stripping operation have concentrated either on long residence time as in U.S. Pat. No. 4,481,103 or extremely high temperatures as in U.S. Pat. No. 4,424,116. These attempts have not been entirely satisfactory.